1. Field of the Invention
The present invention generally relates to a fuel cell system.
2. Background Information
In some conventional fuel cell systems, when the power generation ability of the fuel cells is low, control is carried out to shorten the power generation off time of the fuel cells (see: Japanese Laid-Open Patent Application No. 2006-278276). According to Japanese Laid-Open Patent Application No. 2006-278276, the following is disclosed: when the nitrogen concentration at the anode of a fuel cell stack is high (when the hydrogen concentration at the anode is low), when the operation is restarted, power generation becomes difficult to perform. Consequently, in order to prevent a further increase in the nitrogen concentration due to a cross leak of nitrogen from the cathode while the power generation is off, based on the impurity concentration in the upstream and downstream of the stack while off, the operation off time is set shorter when the impurity concentration is higher. Also, a disclosure is made that the operation re-shutdown prohibition time is set longer.